One of the major problems with converters of the kind described implemented in monolithic (integrated circuit) form results from inherent errors within the components of the circuit. For example, the sample and hold stage generally incorporates a solid state switch to enable the input analogue signal to be samples. When such a switch is operated there is always charge breakthrough from the switch logic drive to the hold capacitor. This can be a very serious source of error and efforts are made to minimise it. These usually require the provision of very good switches (e.g. MOSFET or JFET) and a large hold capacitor. Since bipolar IC processes are more suited to high speed analogue circuitry, the requirement for FET devices complicates the process. Furthermore, since the hold capacitor needs to be large, it must either be sited off the chip on which the other components are provided or it will occupy a very large area of the chip. A further problem is that any input bias current required by the output amplifier of the sample and hold circuit discharges the hold capacitor and so the output "droops" with time. Another problem is that conventional analogue to digital conversion stages, for example flash converters, have a rather different structure from the sample and hold circuit leading to difficulties with variations in processing times.
Problems have also arisen with the second analogue to digital conversion stage. Conventionally, these have made use of high gain amplifiers with feed back around them to provide the necessary gain. While the signal is being acquired for the first conversion stage, the inputs to this feed back amplifier may be as large as the full input range of the converter, consequently the output of the amplifier will be driven into saturation. Gain switching and clamping stages are usually employed with these amplifiers to avoid this.
EP-A-0311105 describes an analogue to digital converter which includes a flash converter but which operates on a feed back principle and suffers from the problems described above in connection with inherent errors. WO-A-90/03066 also discloses a flash converter but again this suffers from the disadvantages described above.